1. Field of the Invention
The present invention relates to an ink-jet printhead. More particularly, the present invention relates to a thermally-driven, monolithic ink-jet printhead, in which a plurality of nozzles is densely disposed to implement high-resolution printing, and a method of manufacturing the same.
2. Description of the Related Art
In general, ink-jet printheads are devices for printing a predetermined image, color or black, by ejecting a small volume droplet of ink at a desired position on a recording sheet. Ink-jet printheads are generally categorized into two types depending on which ink ejection mechanism is used. A first type is a thermally-driven ink-jet printhead, in which a heat source is employed to form and expand a bubble in ink to cause an ink droplet to be ejected due to an expansive force of the formed bubble. A second type is a piezoelectrically-driven ink-jet printhead, in which an ink droplet is ejected by a pressure applied to the ink and a change in ink volume due to a deformation of a piezoelectric element.
An ink droplet ejection mechanism of a thermally-driven ink-jet printhead will now be explained in detail. When a pulse current is supplied to a heater formed of a resistive heating material, the heater generates heat and ink near the heater is instantaneously heated to boiling. The boiling of the ink causes bubbles to be generated, thereby expanding and exerting pressure on the ink filling an ink chamber. As a result, ink in a vicinity of a nozzle is ejected from the ink chamber in the form of a droplet.
A thermal ink-jet printhead is classified into a top-shooting type, a side-shooting type, and a back-shooting type, depending on a growth direction of a bubble and an ejection direction of a droplet. In a top-shooting type printhead, a bubble grows in the same direction in which an ink droplet is ejected. In a side-shooting type of printhead, a bubble grows in a direction perpendicular to a direction in which an ink droplet is ejected. In a back-shooting type of printhead, a bubble grows in a direction opposite to a direction in which an ink droplet is ejected.
An ink-jet printhead using the thermal driving method should satisfy the following requirements. First, manufacturing of the ink-jet printheads should be simple, costs should be low, and should facilitate mass production thereof. Second, in order to obtain a high-quality image, cross talk between adjacent nozzles should be suppressed while a distance between adjacent nozzles should be narrow; that is, in order to increase dots per inch (DPI), a plurality of nozzles should be densely positioned. Third, in order to perform a high-speed printing operation, a period in which the ink chamber is refilled with ink after being ejected from the ink chamber should be as short as possible and the cooling of heated ink and heater should be performed quickly to increase an operating frequency.
FIGS. 1 through 3 illustrate various structures of conventional thermal ink-jet printheads using the back-shooting method.
FIG. 1 illustrates a perspective view of a structure of a first conventional ink-jet printhead. Referring to FIG. 1, an ink-jet printhead 20 includes a substrate 11, a cover plate 3, and an ink reservoir 12. The substrate 11 has a plurality of nozzles 10 through which ink droplets are ejected and an ink chamber 16 filled with ink to be ejected. The cover plate 3 has a through hole 2 providing flow communication between the ink chamber 16 and the ink reservoir 12, which supplies ink to the ink chamber 16. In addition, a heater 22, having a ring shape, is disposed around the nozzle 10 of the substrate 11.
In the above structure, if a pulse current is supplied to the heater 22 and heat is generated by the heater 22, ink in the ink chamber 16 boils and bubbles are generated and continuously expand. Due to this expansion, pressure is applied to ink filling the ink chamber 16. As a result, ink is ejected in droplet form through each of the plurality of nozzles 10. Subsequently, ink flows into the ink chamber 16 from the ink reservoir 12 through the through hole 2 formed in the cover plate 3. Thus, the ink chamber 16 is refilled with ink.
In this first conventional ink-jet printhead 20, however, a depth of the ink chamber 16 is almost the same as a thickness of the substrate 11. Thus, unless a very thin substrate is used, the size of the ink chamber 16 increases. Accordingly, pressure generated by bubbles for ejecting ink is dispersed by the ink, resulting in degradation to an ejection performance. When a thin substrate is used to reduce the size of the ink chamber 16, it becomes more difficult to process the substrate 11. By way of example, a depth of the ink chamber 16 in a typical conventional ink-jet printhead is about 10-30 μm. In order to form an ink chamber having this depth, a silicon substrate having a thickness of 10-30 μm should be used. It is virtually impossible, however, to process a silicon substrate having such a thickness using existing semiconductor processes.
Further, in order to manufacture an ink-jet printhead 20 having the above structure, the substrate 11, the cover plate 3, and the ink reservoir 12 are bonded together. Thus, a process of manufacturing such an ink-jet printhead becomes complicated, and an ink passage which significantly affects an ejection property, cannot be very elaborate due to potential misalignment during the bonding process.
FIGS. 2A and 2B illustrate a structure of a second conventional monolithic ink-jet printhead. More specifically, FIGS. 2A and 2B illustrate a plan view and a vertical cross-sectional view taken along line A-A′ of FIG. 2A, respectively. Referring to FIGS. 2A and 2B, a hemispherical ink chamber 32 is formed on a front surface of a silicon substrate 30. A manifold 36, which supplies ink to the ink chamber 32, is formed on a rear surface of the substrate 30. An ink channel 34, which provides flow communication between the ink chamber 32 and the manifold 36 is formed at a bottom of the ink chamber 32. A nozzle plate 40, in which a plurality of material layers 41, 42, and 43 are stacked, is formed integrally with the substrate 30. A nozzle 47 is formed at a position of the nozzle plate 40 corresponding to a center of the ink chamber 32. A heater 45, which is connected to a conductor 46, is disposed around the nozzle 47. A nozzle guide 44 that extends in a lengthwise direction of the ink chamber 32 is formed at edges of the nozzle 47. In operation, heat generated by the heater 45 is transferred to ink 48 in the ink chamber 32 through an insulating layer 41. As a result, the ink 48 boils, and bubbles 49 are generated in the ink 48. The bubbles 49 expand, and pressure is applied to the ink 48 within the ink chamber 32. As a result, ink 48 in a vicinity of the nozzle 47 is ejected in the form of an ink droplet 48′ through the nozzle 47. Subsequently, due to a surface tension that acts on the surface of the ink 48 contacting air, ink 48 flows into the ink chamber 32 through the ink channel 34 from the manifold 36, thereby refilling the ink chamber 32 with ink 48.
In this second conventional monolithic ink-jet printhead having the above structure, the silicon substrate 30 and the nozzle plate 40 form a single body such that a process of manufacturing the ink-jet printhead is simplified and misalignment may be prevented.
In this configuration, however, in order to form the ink chamber 32, the substrate 30 is isotropically etched through the nozzle 47. As a result, the ink chamber 32 has a hemispherical shape. Thus, in order to form an ink chamber 32 having a predetermined volume, a constant radius of the ink chamber 32 should be maintained. As a result, there is a limitation in narrowing a distance between adjacent nozzles 47 and disposing the nozzles 47 more densely. More specifically, in order to narrow a distance between adjacent nozzles 47, a radius of the ink chamber 32 should be reduced. Such a reduction results in a decrease in a volume of the ink chamber 32, and such a decrease is not preferable.
Accordingly, there is a limitation in densely disposing a plurality of nozzles using the structure of the second conventional monolithic ink-jet printhead, with respect to meeting the requirement for the ink-jet printhead with high DPI to print an image with high-resolution.
FIG. 3 illustrates a structure of a third conventional ink-jet printhead. Referring to FIG. 3, the ink-jet printhead includes a nozzle plate 50 having a nozzle 51, an insulating layer 60 having an ink chamber 61 and an ink channel 62, and a silicon substrate 70 having a manifold 55 for supplying ink to the ink chamber 61. The nozzle plate 50, the insulating layer 60, and the silicon substrate 70 are sequentially stacked.
In this third conventional ink-jet printhead, since the ink chamber 61 is formed using the insulating layer 60 stacked on the substrate 70, the ink chamber 61 may have a variety of shapes, and a backflow of ink may be suppressed.
When manufacturing this third conventional ink-jet printhead, however, a method of depositing the thick insulating layer 60 on the silicon substrate 70, etching the insulating layer 60, and forming the ink chamber 61 is generally used. This method has the following problems. First, it is difficult to stack a thick insulating layer on a substrate using existing semiconductor processes. Second, it is difficult to etch a thick insulating layer. Thus, there is a limitation on the depth of the ink chamber. As shown in FIG. 3, the ink chamber 61 and the nozzle 51 have a combined height of only about 6 μm. With such a shallow ink chamber, however, it is virtually impossible for an ink-jet printhead to have a relatively large drop size.